ABSTRACT Congenital heart disease is the leading cause of death in the western world affecting approximately 1.3 million Americans. The heart initially forms in vertebrates as a bilaminar tube comprised of an inner endocardium and outer myocardial layer. At later stages of development a third layer is added to the heart from the epicardium which forms from a dynamic precursor structure, the proepicardial organ (PEO) which forms on the septum transversum, a structure adjacent to the heart. As the embryo matures, cells from the PEO migrate onto the heart surface ultimately giving rise to several essential cell types in the adult heart including cardiac fibroblasts and the smooth muscle cells of the vasculature. The aim of this proposal will leverage our series of unique technologies, reagents, and animal models to elucidate the molecular and cellular pathways required for the formation and function of the epicardium. In doing so, we will provide a platform for studying heart formation and hemostasis, and thus proved mechanistic insight into human congenital heart disease.